42nd Annual Wooden Bridge Competition

February 19, 2018

Check-in begins at 4:30PM, bridge loading at 5PM. The competition will be held in the Comstock Memorial Ballroom in the Comstock Memorial Union, at the corner of 14th St S and 6th Ave S, on the campus of Minnesota State University Moorhead.

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The Challenge

The competition is open to all students grades 1 through college. Students are invited to build a structure using only wood and glue that:

Spans more than 56 centimeters.

Stands no more than 30 cm tall.

You'll load your bridge with weights. When the bridge breaks, we divide the load it held by the weight of the bridge to determine its strength. The strongest bridges win cash prizes!

Rules

PLTW-friendly rules!

Entrants

The contest is open to any student, grades 1 through 12, plus students enrolled full-time in college. Entrants will be assigned an entry class:

Elementary (Grades 1-6)

Middle School (Grades 7-9)

High School (Grades 10-12)

College

Bridges built by teams of students will be assigned the class of the student in the highest grade.

Design Constraints

Mass

Any bridge mass is allowed. However, lighter bridges will score better in the competition than heavier bridges.

Span

Each structure must span at least 56 cm clear of any supports; in other words, the bridge must be at least 56 cm long. At entry each bridge will be placed on a pair of supports that are 56 center of mass apart. Bridges that do not meet the criteria may still participate but will not be considered for prizes.

Height

The overall height of the structure may not exceed 30 cm.

Wood

The bridge must be constructed of wood. Any kind of wood can be used, but lighter woods like balsa or toothpicks will make lighter bridges. Exception: No fabricated wood products, like particleboard or plywood can be used. Bamboo, which is a not a wood, may not be used.

Adhesive/glue

Any type of glue can be used at joints. No tape or other adhesives may be used.

Coatings

Structures may not be painted, sprayed or dipped in any coating material. Glue may not be used as a coating.

Compiance

At check-in, judges from the Fargo-Moorhead Engineer's Club will evaluate each bridge. Bridges will be weighed, measured, and reviewed for compliance with rules regarding materials. If a bridge does not comply with any of the rules, it will be disqualified from competing for the prizes, but may still be loaded to determine its strength. The decisions of the check-in judges are final.

Supports

The structure will be placed in or on supports made from two 2x4's as shown in the illustration. Structures with no arch in the bottom chord will be placed on the supports to achieve the required minimum span. No lateral or side support is allowed.

Loading

Round metal weights, approximately 220 mm in diameter, will be placed on the top of the structure at the mid-span by the contestants until failure. A 160-mm round wooden disk will be available to place between the structure and the weights, if desired. Contestants may use cards to shim the base of the bridge level. Entrants in the elementary class may ask for assistance in loading their structures.

Scoring

Judges from the Fargo-Moorhead Engineers Club will record the total weight supported by the bridge. The judge will make the final decision as to whether a structure successfully supported the final weight before failure. The total weight supported will then be divided by the recorded weight of the structure to determine the strength ratio of the bridge.

Prizes

Prize amounts and additional details will be available after Dec. 1.

Reminder: Bridge performance is judged by the weight supported divided by the weight of the bridge. The winning bridge might not be the bridge that supports the most weight.

Grand prize

The best-performing bridge receives a prize of $100

First place in grade level

The best performing bridge in each grade category (elementary, middle school, high school) will receive a prize of $50

Second place in grade level

The second best performing bridge in each category will receive a prize of $40

Third place in grade level

Each third place bridge receives a prize of $30

Best college bridge

The highest-performing bridge entered by a college student will receive a prize of $40

Best appearance

One bridge will be chosen by the entry judges for best appearance and will receive a prize of $40

Tips

Note from the rules that the winner is judged not on the total weight supported, but rather on the capacity of their bridge - weight supported/weight of the bridge. That means that if a bridge is very light and moderately strong, it can win over a stronger, heavier bridge.

Gluing the toothpicks together can take time and is delicate work. Lay out a sheet of wax paper, glue together several toothpicks to form a base plane, and allow that to dry. Then glue members to the base plane in order to create a space frame.

In the past, many people have built flat bridges - mats of toothpicks only 2 or three layers deep, and containing no support shapes. These have never been successful. They bend under even a very small load and split. Create a bridge with some depth - use some method to create a support structure that transfers the weight from the middle of the bridge to the supports.

Some useful shapes to use as building blocks are:

Posts

Glue several toothpicks together side by side to build up a round member the size of a pencil. These members will be very strong in compression, but will still need support to keep them from buckling.

Triangular space frames

Typically, the strongest space frames are made by connecting triangles. You can begin by building a tetrahedron. First, glue together 3 toothpicks to form a triangle. This is your base plane. When it dries, add 3 more toothpicks (one at each corner) to form a 3-sided pyramid, or tetrahedron. You can add to this shape by gluing 3 more toothpicks to any of its triangular faces. The weak point on these is usually the corners - let the toothpicks cross over each other slightly, and cover the joint in glue.

Trusses

Trusses are like the rafters in a roof, or the typical steel bridge. Trusses are usually built by connecting a series of triangles in a plane. Bridges can then be built by setting two trusses parallel to each other and connecting them with cross members.